An Optical Transport Network (OTN) is defined by ITU-T as a set of optical elements that are connected by optical links and that are capable of providing transporting, multiplexing, switching, management, and supervision functionality, and survivability of optical channels carrying client signals.
An OTN uses digital wrapper technology that provides a method for encapsulating data in an optical data unit. Generally, digital wrapping involves grouping a number of existing optical data units together into one entity that can be more efficiently managed with a small allocation of overhead and forward error correction (FEC) bytes. There are multiple levels of hierarchy for encapsulation, including an Optical channel Payload Unit (OPU) for encapsulation of client data, an Optical channel Data Unit (ODU) for encapsulating the OPU data, an Optical channel Transport Unit (OTU) for encapsulating the ODU data.
A higher order (HO) ODU flow may hierarchically be comprised of one or more lower order (LO) ODU flows multiplexed into the HO ODU flow. For example, an ODU4 flow, a particular HO ODU, may include a combination of LO ODU flows (e.g., ODU0, ODU1, ODU2, ODU2e, ODU3, ODU3e, ODU3e2, ODUflex, etc.) occupying a set of byte slots associated with the ODU4 flow. Frames of an ODU flow may be identified by frame alignment signal (FAS) data. Consequently, an optical receiver may include a framer circuit to detect FAS data for each LO ODU flow in order to read information from the LO ODU flows.